After the American Academy of Pediatrics announced its support for equal marriage rights for same-sex partners "as the best way to guarantee benefits and security for their children," advocates for the unmarried, while applauding the move, took exception to the language.

In a petition posted on change.org, the group Unmarried Equality (unmarried.org) asked the pediatrics group to modify its statement to recognize that unmarried parents are just as capable of providing stable homes for their kids and that a marriage license does not a good parent make.

The academy's policy statement does not suggest that married parents are better parents, but rather that marriage provides the optimal legal and social protections for raising kids, and that couples who wish to marry should be able to do so regardless of their sexual orientation.

But with 35 percent of children not being raised by married parents living together, the challenge raises the question: Does parental marital status matter to a child's well-being?

Studies have produced consistent evidence that children reared by two biological married parents fare better than those raised in alternative arrangements, but the difference is small and explained mostly by differences in parenting style and availability of resources rather than some magic conferred by the marriage license itself, said Susan Brown, sociology professor and co-director of the National Center for Family and Marriage Research at Bowling Green State University in Ohio. Couples who don't get married tend to be more economically disadvantaged. (These differences have not been found among gay parents because, for most, marriage has not been an option, Brown said.)

Still, even when studies adjust for finances and other factors, the children of married parents have been shown to have slightly better outcomes — as reflected in performance in school, delinquency, psychological distress, anxiety and physical health — than children raised by cohabiting unmarried parents, Brown said.

That may be because unmarried couples can be less stable, causing everyone stress, and they're more likely to have shifting family arrangements. Again, that's a function of the quality of the relationship rather than a marriage license, as married couples who always fight have also been shown to hurt a child's well-being.

"There seems to be mounting evidence that family instability, as opposed to family structure itself, is particularly detrimental to children," Brown said.

Dr. Benjamin Siegel, a pediatrician at Boston Medical Center and chair of the AAP Committee on Psychosocial Aspects of Child and Family Health, stressed that what's important isn't marriage per se, but a stable, good-quality, secure relationship with two adults, and often marriage is a good way to achieve that.

But the marriage license itself is also important: It confers financial and legal protections to the family, including spousal maintenance if the couple splits. It can be especially important for nonbiological parents, who in many states have no custody and visitation rights or child-support obligations if the relationship ends, said Siegel, co-author of the policy statement.

"Anyone who is (not married and) raising a child should develop a legal mechanism so that person has a legal relationship to the child, whether it's adoption or guardianship," Siegel said.

The system is set up to favor marriage, which automatically flows various legal protections through the family that benefit children, such as health care, inheritance, Social Security, medical decisions, and family and medical leave, said Alan Boudreau, vice chair of the alternative families committee for the American Bar Association's Section of Family Law. Unmarried parents must go through additional processes, which take time and cost money, to secure the same benefits.

That system is what Unmarried Equality, in its petition, denounces as "rampant marital status discrimination."

For better or worse, it is, for now, the reality.

"Families have become more complex, and the law is still catching up," Boudreau said.